Not necessarily open source, but where there is good, honest ad/spyware free freeware, Why buy? - the arena belongs to freeware.
I'll take full coverCD versions as well.
http://www.eudora.com/email/modes/
Eudora is another possibilty, with Lite(free), Adware and paid modes - which leads to an immediate distrust as to how well the adware components are suppressed in Lite and Paid modes!
Paid mode is $49.95, and enables the spam filter, but is it REALLY worth $49.95 more than Thunderbird, which also has spam filtering?
I like the name used at one time in alt.comp.freeware for 30 day trials (and shareware/30 day trial)
"Drug dealer ware" - give you a fix until you're hooked, then you PAY.
I don't do trials, EVER - the only exception being to determine if something I would have bought, is actually no good.
Liteware versions, I'll use as is, and therfore they MIGHT get an upgrade out of me at some point.
PS. Thunderbird has now gone to 0.6 RC2
http://www.mozillazine.org
http://www.mozilla.org/products/thunderbird/releases/